1. Field of the Invention
The present invention is directed to a computer tomography apparatus of the type wherein a spiral scan of an examination subject is undertaken.
2. Description of the Prior Art
Computer tomography systems are known wherein an examination subject is three-dimensionally scanned on the basis of relative motion between the measurement unit, consisting of an x-ray radiator and a radiation detector, and the patient support, with the patient thereon. The relative motion ensues in the longitudinal direction of the patient support during the acquisition and processing of the detector data, with the measurement unit rotating around the examination subject.
A scan of this type is known as a spiral scan. When registering the data for such spiral scans, the measured data arise at different z-positions (spiral data). The z-coordinate characterizes the relative position of the measured slice vis-a-vis the examination subject. The z-direction is thus oriented perpendicularly to the measured slice. The conventional tomographic reconstruction algorithm, however, employs data obtained for a constant z-position. For systems employing a spiral scan, therefore, interpolation algorithms have been developed which generate new data, by interpolation, corresponding to a planar slice from the spiral data before the actual image reconstruction. The goal of the interpolation is that the interpolated data are as close as possible to those which would actually be measured in a planar slice. Interpolation algorithms hitherto employed have always operated on the data in the form of attenuation values. Attenuation values are scaled line integrals or scaled logarithms of the relative intensities.
If a partial volume effect (i.e. a few objects of interest (organs) only partially project into the measured slice) is present, however, a problem arises in this known type of interpolation. In comparison to a true planar scan, the levels of the CT values of the affected objects and their contours are modified, and images which should actually be rotationally-symmetrical no longer appear symmetrical.